1. Field of the Invention
The present invention relates to an apparatus for generating a train of light pulses and more particularly to a practical apparatus of generating a light pulse train, having a simple structure and capable of generating a train of pulses having a high repetition rate, in which the ends of a length of optical fiber are coupled to each other to form a closed loop so that light wave may propagate freely through the looped fiber.
2. Description of the Prior Art
As a source of light pulses are known a variety of conventional discharge tubes. With these discharge tubes, the input current is periodically interrupted to generate light pulses. The light pulses thus produced, however, are incoherent, that is consist of light components having various wavelengths, and have a long pulse duration or width and a low repetition frequency, so that the tubes are used only with practical limitations, regarded as improper for optical communications especially designed for very wide band. Through the use of laser apparatus, a train of pulses having an excellent coherency, a short pulse width and a high repetition rate can be obtained. This can be realized by inserting a modulator element in the laser resonator and by performing amplitude modulation with a frequency equivalent to the length of the longitudinal mode of the resonator. The laser system so equipped as to perform such a procedure is called a mode-locked laser. According to this technique, an ideal train of pulses having a very short width and a very high repetition rate can be obtained indeed, but it is not practical in several aspects to use this artifice with the conventional parts or apparatuses. Namely, if it is used with the conventional devices, various difficulties are caused: the over-all structure is too big when it is used in a system where a plurality of mirrors are used to form a loop for light, very severe conditions are imposed upon the mechanical precision and stability of the structure of the frequency stabilizing device, and a very complicated electronic system is required for drive stabilization. Recently, therefore, the use of a semiconductor laser apparatus or a light emitting diode as a light source for generating a train of light pulses used in optical communication, has been proposed since such an apparatus or diode leads to the reduction of size, the improvement in efficiency and the increase in repetition frequency. However, numerous problems still remain to be solved with these light pulse generators.